From the driver comments :
"Obtains a waveform measurement of the specified measurement slot and returns the measurement value."
So it is choosing from which slot you are getting the data. I suppose that "Immediate" would use the present channel that is already selected in the isntrument.
If you have the programming manual check at this : *WAI;:MEASU:%s:VAL? where %s is replaced by the choosen channel.
